/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea and Martin Buchholz with assistance from members of
 * JCP JSR-166 Expert Group and released to the public domain, as explained
 * at http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.AbstractQueue;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;

/**
 * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
 * This queue orders elements FIFO (first-in-first-out).
 * The <em>head</em> of the queue is that element that has been on the
 * queue the longest time.
 * The <em>tail</em> of the queue is that element that has been on the
 * queue the shortest time. New elements
 * are inserted at the tail of the queue, and the queue retrieval
 * operations obtain elements at the head of the queue.
 * A {@code ConcurrentLinkedQueue} is an appropriate choice when
 * many threads will share access to a common collection.
 * Like most other concurrent collection implementations, this class
 * does not permit the use of {@code null} elements.
 *
 * <p>This implementation employs an efficient <em>non-blocking</em>
 * algorithm based on one described in <a
 * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
 * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
 * Algorithms</a> by Maged M. Michael and Michael L. Scott.
 *
 * <p>Iterators are <i>weakly consistent</i>, returning elements
 * reflecting the state of the queue at some point at or since the
 * creation of the iterator.  They do <em>not</em> throw {@link
 * java.util.ConcurrentModificationException}, and may proceed concurrently
 * with other operations.  Elements contained in the queue since the creation
 * of the iterator will be returned exactly once.
 *
 * <p>Beware that, unlike in most collections, the {@code size} method
 * is <em>NOT</em> a constant-time operation. Because of the
 * asynchronous nature of these queues, determining the current number
 * of elements requires a traversal of the elements, and so may report
 * inaccurate results if this collection is modified during traversal.
 * Additionally, the bulk operations {@code addAll},
 * {@code removeAll}, {@code retainAll}, {@code containsAll},
 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
 * to be performed atomically. For example, an iterator operating
 * concurrently with an {@code addAll} operation might view only some
 * of the added elements.
 *
 * <p>This class and its iterator implement all of the <em>optional</em>
 * methods of the {@link Queue} and {@link Iterator} interfaces.
 *
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code ConcurrentLinkedQueue}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that element from
 * the {@code ConcurrentLinkedQueue} in another thread.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this collection
 * @author Doug Lea
 * @since 1.5
 */
public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
    implements Queue<E>, java.io.Serializable {

  private static final long serialVersionUID = 196745693267521676L;

    /*
     * This is a modification of the Michael & Scott algorithm,
     * adapted for a garbage-collected environment, with support for
     * interior node deletion (to support remove(Object)).  For
     * explanation, read the paper.
     *
     * Note that like most non-blocking algorithms in this package,
     * this implementation relies on the fact that in garbage
     * collected systems, there is no possibility of ABA problems due
     * to recycled nodes, so there is no need to use "counted
     * pointers" or related techniques seen in versions used in
     * non-GC'ed settings.
     *
     * The fundamental invariants are:
     * - There is exactly one (last) Node with a null next reference,
     *   which is CASed when enqueueing.  This last Node can be
     *   reached in O(1) time from tail, but tail is merely an
     *   optimization - it can always be reached in O(N) time from
     *   head as well.
     * - The elements contained in the queue are the non-null items in
     *   Nodes that are reachable from head.  CASing the item
     *   reference of a Node to null atomically removes it from the
     *   queue.  Reachability of all elements from head must remain
     *   true even in the case of concurrent modifications that cause
     *   head to advance.  A dequeued Node may remain in use
     *   indefinitely due to creation of an Iterator or simply a
     *   poll() that has lost its time slice.
     *
     * The above might appear to imply that all Nodes are GC-reachable
     * from a predecessor dequeued Node.  That would cause two problems:
     * - allow a rogue Iterator to cause unbounded memory retention
     * - cause cross-generational linking of old Nodes to new Nodes if
     *   a Node was tenured while live, which generational GCs have a
     *   hard time dealing with, causing repeated major collections.
     * However, only non-deleted Nodes need to be reachable from
     * dequeued Nodes, and reachability does not necessarily have to
     * be of the kind understood by the GC.  We use the trick of
     * linking a Node that has just been dequeued to itself.  Such a
     * self-link implicitly means to advance to head.
     *
     * Both head and tail are permitted to lag.  In fact, failing to
     * update them every time one could is a significant optimization
     * (fewer CASes). As with LinkedTransferQueue (see the internal
     * documentation for that class), we use a slack threshold of two;
     * that is, we update head/tail when the current pointer appears
     * to be two or more steps away from the first/last node.
     *
     * Since head and tail are updated concurrently and independently,
     * it is possible for tail to lag behind head (why not)?
     *
     * CASing a Node's item reference to null atomically removes the
     * element from the queue.  Iterators skip over Nodes with null
     * items.  Prior implementations of this class had a race between
     * poll() and remove(Object) where the same element would appear
     * to be successfully removed by two concurrent operations.  The
     * method remove(Object) also lazily unlinks deleted Nodes, but
     * this is merely an optimization.
     *
     * When constructing a Node (before enqueuing it) we avoid paying
     * for a volatile write to item by using Unsafe.putObject instead
     * of a normal write.  This allows the cost of enqueue to be
     * "one-and-a-half" CASes.
     *
     * Both head and tail may or may not point to a Node with a
     * non-null item.  If the queue is empty, all items must of course
     * be null.  Upon creation, both head and tail refer to a dummy
     * Node with null item.  Both head and tail are only updated using
     * CAS, so they never regress, although again this is merely an
     * optimization.
     */

  private static class Node<E> {

    volatile E item;
    volatile Node<E> next;

    /**
     * Constructs a new node.  Uses relaxed write because item can
     * only be seen after publication via casNext.
     */
    Node(E item) {
      UNSAFE.putObject(this, itemOffset, item);
    }

    boolean casItem(E cmp, E val) {
      return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
    }

    void lazySetNext(Node<E> val) {
      UNSAFE.putOrderedObject(this, nextOffset, val);
    }

    boolean casNext(Node<E> cmp, Node<E> val) {
      return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE;
    private static final long itemOffset;
    private static final long nextOffset;

    static {
      try {
        UNSAFE = sun.misc.Unsafe.getUnsafe();
        Class<?> k = Node.class;
        itemOffset = UNSAFE.objectFieldOffset
            (k.getDeclaredField("item"));
        nextOffset = UNSAFE.objectFieldOffset
            (k.getDeclaredField("next"));
      } catch (Exception e) {
        throw new Error(e);
      }
    }
  }

  /**
   * A node from which the first live (non-deleted) node (if any)
   * can be reached in O(1) time.
   * Invariants:
   * - all live nodes are reachable from head via succ()
   * - head != null
   * - (tmp = head).next != tmp || tmp != head
   * Non-invariants:
   * - head.item may or may not be null.
   * - it is permitted for tail to lag behind head, that is, for tail
   * to not be reachable from head!
   */
  private transient volatile Node<E> head;

  /**
   * A node from which the last node on list (that is, the unique
   * node with node.next == null) can be reached in O(1) time.
   * Invariants:
   * - the last node is always reachable from tail via succ()
   * - tail != null
   * Non-invariants:
   * - tail.item may or may not be null.
   * - it is permitted for tail to lag behind head, that is, for tail
   * to not be reachable from head!
   * - tail.next may or may not be self-pointing to tail.
   */
  private transient volatile Node<E> tail;

  /**
   * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
   */
  public ConcurrentLinkedQueue() {
    head = tail = new Node<E>(null);
  }

  /**
   * Creates a {@code ConcurrentLinkedQueue}
   * initially containing the elements of the given collection,
   * added in traversal order of the collection's iterator.
   *
   * @param c the collection of elements to initially contain
   * @throws NullPointerException if the specified collection or any of its elements are null
   */
  public ConcurrentLinkedQueue(Collection<? extends E> c) {
    Node<E> h = null, t = null;
    for (E e : c) {
      checkNotNull(e);
      Node<E> newNode = new Node<E>(e);
      if (h == null) {
        h = t = newNode;
      } else {
        t.lazySetNext(newNode);
        t = newNode;
      }
    }
    if (h == null) {
      h = t = new Node<E>(null);
    }
    head = h;
    tail = t;
  }

  // Have to override just to update the javadoc

  /**
   * Inserts the specified element at the tail of this queue.
   * As the queue is unbounded, this method will never throw
   * {@link IllegalStateException} or return {@code false}.
   *
   * @return {@code true} (as specified by {@link Collection#add})
   * @throws NullPointerException if the specified element is null
   */
  public boolean add(E e) {
    return offer(e);
  }

  /**
   * Tries to CAS head to p. If successful, repoint old head to itself
   * as sentinel for succ(), below.
   */
  final void updateHead(Node<E> h, Node<E> p) {
    if (h != p && casHead(h, p)) {
      h.lazySetNext(h);
    }
  }

  /**
   * Returns the successor of p, or the head node if p.next has been
   * linked to self, which will only be true if traversing with a
   * stale pointer that is now off the list.
   */
  final Node<E> succ(Node<E> p) {
    Node<E> next = p.next;
    return (p == next) ? head : next;
  }

  /**
   * Inserts the specified element at the tail of this queue.
   * As the queue is unbounded, this method will never return {@code false}.
   *
   * @return {@code true} (as specified by {@link Queue#offer})
   * @throws NullPointerException if the specified element is null
   */
  public boolean offer(E e) {
    checkNotNull(e);
    final Node<E> newNode = new Node<E>(e);

    for (Node<E> t = tail, p = t; ; ) {
      Node<E> q = p.next;
      if (q == null) {
        // p is last node
        if (p.casNext(null, newNode)) {
          // Successful CAS is the linearization point
          // for e to become an element of this queue,
          // and for newNode to become "live".
          if (p != t) // hop two nodes at a time
          {
            casTail(t, newNode);  // Failure is OK.
          }
          return true;
        }
        // Lost CAS race to another thread; re-read next
      } else if (p == q)
      // We have fallen off list.  If tail is unchanged, it
      // will also be off-list, in which case we need to
      // jump to head, from which all live nodes are always
      // reachable.  Else the new tail is a better bet.
      {
        p = (t != (t = tail)) ? t : head;
      } else
      // Check for tail updates after two hops.
      {
        p = (p != t && t != (t = tail)) ? t : q;
      }
    }
  }

  public E poll() {
    restartFromHead:
    for (; ; ) {
      for (Node<E> h = head, p = h, q; ; ) {
        E item = p.item;

        if (item != null && p.casItem(item, null)) {
          // Successful CAS is the linearization point
          // for item to be removed from this queue.
          if (p != h) // hop two nodes at a time
          {
            updateHead(h, ((q = p.next) != null) ? q : p);
          }
          return item;
        } else if ((q = p.next) == null) {
          updateHead(h, p);
          return null;
        } else if (p == q) {
          continue restartFromHead;
        } else {
          p = q;
        }
      }
    }
  }

  public E peek() {
    restartFromHead:
    for (; ; ) {
      for (Node<E> h = head, p = h, q; ; ) {
        E item = p.item;
        if (item != null || (q = p.next) == null) {
          updateHead(h, p);
          return item;
        } else if (p == q) {
          continue restartFromHead;
        } else {
          p = q;
        }
      }
    }
  }

  /**
   * Returns the first live (non-deleted) node on list, or null if none.
   * This is yet another variant of poll/peek; here returning the
   * first node, not element.  We could make peek() a wrapper around
   * first(), but that would cost an extra volatile read of item,
   * and the need to add a retry loop to deal with the possibility
   * of losing a race to a concurrent poll().
   */
  Node<E> first() {
    restartFromHead:
    for (; ; ) {
      for (Node<E> h = head, p = h, q; ; ) {
        boolean hasItem = (p.item != null);
        if (hasItem || (q = p.next) == null) {
          updateHead(h, p);
          return hasItem ? p : null;
        } else if (p == q) {
          continue restartFromHead;
        } else {
          p = q;
        }
      }
    }
  }

  /**
   * Returns {@code true} if this queue contains no elements.
   *
   * @return {@code true} if this queue contains no elements
   */
  public boolean isEmpty() {
    return first() == null;
  }

  /**
   * Returns the number of elements in this queue.  If this queue
   * contains more than {@code Integer.MAX_VALUE} elements, returns
   * {@code Integer.MAX_VALUE}.
   *
   * <p>Beware that, unlike in most collections, this method is
   * <em>NOT</em> a constant-time operation. Because of the
   * asynchronous nature of these queues, determining the current
   * number of elements requires an O(n) traversal.
   * Additionally, if elements are added or removed during execution
   * of this method, the returned result may be inaccurate.  Thus,
   * this method is typically not very useful in concurrent
   * applications.
   *
   * @return the number of elements in this queue
   */
  public int size() {
    int count = 0;
    for (Node<E> p = first(); p != null; p = succ(p)) {
      if (p.item != null)
      // Collection.size() spec says to max out
      {
        if (++count == Integer.MAX_VALUE) {
          break;
        }
      }
    }
    return count;
  }

  /**
   * Returns {@code true} if this queue contains the specified element.
   * More formally, returns {@code true} if and only if this queue contains
   * at least one element {@code e} such that {@code o.equals(e)}.
   *
   * @param o object to be checked for containment in this queue
   * @return {@code true} if this queue contains the specified element
   */
  public boolean contains(Object o) {
    if (o == null) {
      return false;
    }
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null && o.equals(item)) {
        return true;
      }
    }
    return false;
  }

  /**
   * Removes a single instance of the specified element from this queue,
   * if it is present.  More formally, removes an element {@code e} such
   * that {@code o.equals(e)}, if this queue contains one or more such
   * elements.
   * Returns {@code true} if this queue contained the specified element
   * (or equivalently, if this queue changed as a result of the call).
   *
   * @param o element to be removed from this queue, if present
   * @return {@code true} if this queue changed as a result of the call
   */
  public boolean remove(Object o) {
    if (o == null) {
      return false;
    }
    Node<E> pred = null;
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null &&
          o.equals(item) &&
          p.casItem(item, null)) {
        Node<E> next = succ(p);
        if (pred != null && next != null) {
          pred.casNext(p, next);
        }
        return true;
      }
      pred = p;
    }
    return false;
  }

  /**
   * Appends all of the elements in the specified collection to the end of
   * this queue, in the order that they are returned by the specified
   * collection's iterator.  Attempts to {@code addAll} of a queue to
   * itself result in {@code IllegalArgumentException}.
   *
   * @param c the elements to be inserted into this queue
   * @return {@code true} if this queue changed as a result of the call
   * @throws NullPointerException if the specified collection or any of its elements are null
   * @throws IllegalArgumentException if the collection is this queue
   */
  public boolean addAll(Collection<? extends E> c) {
    if (c == this)
    // As historically specified in AbstractQueue#addAll
    {
      throw new IllegalArgumentException();
    }

    // Copy c into a private chain of Nodes
    Node<E> beginningOfTheEnd = null, last = null;
    for (E e : c) {
      checkNotNull(e);
      Node<E> newNode = new Node<E>(e);
      if (beginningOfTheEnd == null) {
        beginningOfTheEnd = last = newNode;
      } else {
        last.lazySetNext(newNode);
        last = newNode;
      }
    }
    if (beginningOfTheEnd == null) {
      return false;
    }

    // Atomically append the chain at the tail of this collection
    for (Node<E> t = tail, p = t; ; ) {
      Node<E> q = p.next;
      if (q == null) {
        // p is last node
        if (p.casNext(null, beginningOfTheEnd)) {
          // Successful CAS is the linearization point
          // for all elements to be added to this queue.
          if (!casTail(t, last)) {
            // Try a little harder to update tail,
            // since we may be adding many elements.
            t = tail;
            if (last.next == null) {
              casTail(t, last);
            }
          }
          return true;
        }
        // Lost CAS race to another thread; re-read next
      } else if (p == q)
      // We have fallen off list.  If tail is unchanged, it
      // will also be off-list, in which case we need to
      // jump to head, from which all live nodes are always
      // reachable.  Else the new tail is a better bet.
      {
        p = (t != (t = tail)) ? t : head;
      } else
      // Check for tail updates after two hops.
      {
        p = (p != t && t != (t = tail)) ? t : q;
      }
    }
  }

  /**
   * Returns an array containing all of the elements in this queue, in
   * proper sequence.
   *
   * <p>The returned array will be "safe" in that no references to it are
   * maintained by this queue.  (In other words, this method must allocate
   * a new array).  The caller is thus free to modify the returned array.
   *
   * <p>This method acts as bridge between array-based and collection-based
   * APIs.
   *
   * @return an array containing all of the elements in this queue
   */
  public Object[] toArray() {
    // Use ArrayList to deal with resizing.
    ArrayList<E> al = new ArrayList<E>();
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null) {
        al.add(item);
      }
    }
    return al.toArray();
  }

  /**
   * Returns an array containing all of the elements in this queue, in
   * proper sequence; the runtime type of the returned array is that of
   * the specified array.  If the queue fits in the specified array, it
   * is returned therein.  Otherwise, a new array is allocated with the
   * runtime type of the specified array and the size of this queue.
   *
   * <p>If this queue fits in the specified array with room to spare
   * (i.e., the array has more elements than this queue), the element in
   * the array immediately following the end of the queue is set to
   * {@code null}.
   *
   * <p>Like the {@link #toArray()} method, this method acts as bridge between
   * array-based and collection-based APIs.  Further, this method allows
   * precise control over the runtime type of the output array, and may,
   * under certain circumstances, be used to save allocation costs.
   *
   * <p>Suppose {@code x} is a queue known to contain only strings.
   * The following code can be used to dump the queue into a newly
   * allocated array of {@code String}:
   *
   * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
   *
   * Note that {@code toArray(new Object[0])} is identical in function to
   * {@code toArray()}.
   *
   * @param a the array into which the elements of the queue are to be stored, if it is big enough;
   * otherwise, a new array of the same runtime type is allocated for this purpose
   * @return an array containing all of the elements in this queue
   * @throws ArrayStoreException if the runtime type of the specified array is not a supertype of
   * the runtime type of every element in this queue
   * @throws NullPointerException if the specified array is null
   */
  @SuppressWarnings("unchecked")
  public <T> T[] toArray(T[] a) {
    // try to use sent-in array
    int k = 0;
    Node<E> p;
    for (p = first(); p != null && k < a.length; p = succ(p)) {
      E item = p.item;
      if (item != null) {
        a[k++] = (T) item;
      }
    }
    if (p == null) {
      if (k < a.length) {
        a[k] = null;
      }
      return a;
    }

    // If won't fit, use ArrayList version
    ArrayList<E> al = new ArrayList<E>();
    for (Node<E> q = first(); q != null; q = succ(q)) {
      E item = q.item;
      if (item != null) {
        al.add(item);
      }
    }
    return al.toArray(a);
  }

  /**
   * Returns an iterator over the elements in this queue in proper sequence.
   * The elements will be returned in order from first (head) to last (tail).
   *
   * <p>The returned iterator is
   * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
   *
   * @return an iterator over the elements in this queue in proper sequence
   */
  public Iterator<E> iterator() {
    return new Itr();
  }

  private class Itr implements Iterator<E> {

    /**
     * Next node to return item for.
     */
    private Node<E> nextNode;

    /**
     * nextItem holds on to item fields because once we claim
     * that an element exists in hasNext(), we must return it in
     * the following next() call even if it was in the process of
     * being removed when hasNext() was called.
     */
    private E nextItem;

    /**
     * Node of the last returned item, to support remove.
     */
    private Node<E> lastRet;

    Itr() {
      advance();
    }

    /**
     * Moves to next valid node and returns item to return for
     * next(), or null if no such.
     */
    private E advance() {
      lastRet = nextNode;
      E x = nextItem;

      Node<E> pred, p;
      if (nextNode == null) {
        p = first();
        pred = null;
      } else {
        pred = nextNode;
        p = succ(nextNode);
      }

      for (; ; ) {
        if (p == null) {
          nextNode = null;
          nextItem = null;
          return x;
        }
        E item = p.item;
        if (item != null) {
          nextNode = p;
          nextItem = item;
          return x;
        } else {
          // skip over nulls
          Node<E> next = succ(p);
          if (pred != null && next != null) {
            pred.casNext(p, next);
          }
          p = next;
        }
      }
    }

    public boolean hasNext() {
      return nextNode != null;
    }

    public E next() {
      if (nextNode == null) {
        throw new NoSuchElementException();
      }
      return advance();
    }

    public void remove() {
      Node<E> l = lastRet;
      if (l == null) {
        throw new IllegalStateException();
      }
      // rely on a future traversal to relink.
      l.item = null;
      lastRet = null;
    }
  }

  /**
   * Saves this queue to a stream (that is, serializes it).
   *
   * @param s the stream
   * @throws java.io.IOException if an I/O error occurs
   * @serialData All of the elements (each an {@code E}) in the proper order, followed by a null
   */
  private void writeObject(java.io.ObjectOutputStream s)
      throws java.io.IOException {

    // Write out any hidden stuff
    s.defaultWriteObject();

    // Write out all elements in the proper order.
    for (Node<E> p = first(); p != null; p = succ(p)) {
      Object item = p.item;
      if (item != null) {
        s.writeObject(item);
      }
    }

    // Use trailing null as sentinel
    s.writeObject(null);
  }

  /**
   * Reconstitutes this queue from a stream (that is, deserializes it).
   *
   * @param s the stream
   * @throws ClassNotFoundException if the class of a serialized object could not be found
   * @throws java.io.IOException if an I/O error occurs
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    s.defaultReadObject();

    // Read in elements until trailing null sentinel found
    Node<E> h = null, t = null;
    Object item;
    while ((item = s.readObject()) != null) {
      @SuppressWarnings("unchecked")
      Node<E> newNode = new Node<E>((E) item);
      if (h == null) {
        h = t = newNode;
      } else {
        t.lazySetNext(newNode);
        t = newNode;
      }
    }
    if (h == null) {
      h = t = new Node<E>(null);
    }
    head = h;
    tail = t;
  }

  /**
   * A customized variant of Spliterators.IteratorSpliterator
   */
  static final class CLQSpliterator<E> implements Spliterator<E> {

    static final int MAX_BATCH = 1 << 25;  // max batch array size;
    final ConcurrentLinkedQueue<E> queue;
    Node<E> current;    // current node; null until initialized
    int batch;          // batch size for splits
    boolean exhausted;  // true when no more nodes

    CLQSpliterator(ConcurrentLinkedQueue<E> queue) {
      this.queue = queue;
    }

    public Spliterator<E> trySplit() {
      Node<E> p;
      final ConcurrentLinkedQueue<E> q = this.queue;
      int b = batch;
      int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
      if (!exhausted &&
          ((p = current) != null || (p = q.first()) != null) &&
          p.next != null) {
        Object[] a = new Object[n];
        int i = 0;
        do {
          if ((a[i] = p.item) != null) {
            ++i;
          }
          if (p == (p = p.next)) {
            p = q.first();
          }
        } while (p != null && i < n);
        if ((current = p) == null) {
          exhausted = true;
        }
        if (i > 0) {
          batch = i;
          return Spliterators.spliterator
              (a, 0, i, Spliterator.ORDERED | Spliterator.NONNULL |
                  Spliterator.CONCURRENT);
        }
      }
      return null;
    }

    public void forEachRemaining(Consumer<? super E> action) {
      Node<E> p;
      if (action == null) {
        throw new NullPointerException();
      }
      final ConcurrentLinkedQueue<E> q = this.queue;
      if (!exhausted &&
          ((p = current) != null || (p = q.first()) != null)) {
        exhausted = true;
        do {
          E e = p.item;
          if (p == (p = p.next)) {
            p = q.first();
          }
          if (e != null) {
            action.accept(e);
          }
        } while (p != null);
      }
    }

    public boolean tryAdvance(Consumer<? super E> action) {
      Node<E> p;
      if (action == null) {
        throw new NullPointerException();
      }
      final ConcurrentLinkedQueue<E> q = this.queue;
      if (!exhausted &&
          ((p = current) != null || (p = q.first()) != null)) {
        E e;
        do {
          e = p.item;
          if (p == (p = p.next)) {
            p = q.first();
          }
        } while (e == null && p != null);
        if ((current = p) == null) {
          exhausted = true;
        }
        if (e != null) {
          action.accept(e);
          return true;
        }
      }
      return false;
    }

    public long estimateSize() {
      return Long.MAX_VALUE;
    }

    public int characteristics() {
      return Spliterator.ORDERED | Spliterator.NONNULL |
          Spliterator.CONCURRENT;
    }
  }

  /**
   * Returns a {@link Spliterator} over the elements in this queue.
   *
   * <p>The returned spliterator is
   * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
   *
   * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
   * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
   *
   * @return a {@code Spliterator} over the elements in this queue
   * @implNote The {@code Spliterator} implements {@code trySplit} to permit limited parallelism.
   * @since 1.8
   */
  @Override
  public Spliterator<E> spliterator() {
    return new CLQSpliterator<E>(this);
  }

  /**
   * Throws NullPointerException if argument is null.
   *
   * @param v the element
   */
  private static void checkNotNull(Object v) {
    if (v == null) {
      throw new NullPointerException();
    }
  }

  private boolean casTail(Node<E> cmp, Node<E> val) {
    return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
  }

  private boolean casHead(Node<E> cmp, Node<E> val) {
    return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
  }

  // Unsafe mechanics

  private static final sun.misc.Unsafe UNSAFE;
  private static final long headOffset;
  private static final long tailOffset;

  static {
    try {
      UNSAFE = sun.misc.Unsafe.getUnsafe();
      Class<?> k = ConcurrentLinkedQueue.class;
      headOffset = UNSAFE.objectFieldOffset
          (k.getDeclaredField("head"));
      tailOffset = UNSAFE.objectFieldOffset
          (k.getDeclaredField("tail"));
    } catch (Exception e) {
      throw new Error(e);
    }
  }
}
